Combinations of non-traumatic impulse and continuous noise can produce unusually severe changes in hearing sensitivity and cochlear integrity. The extent of this noise-induced trauma cannot be explained on the basis of addition of the acoustic power of the two noises. The purpose of the proposed research is to systematically study the effects of combined impulse and continuous noise exposure on hearing sensitivity and cochlear anatomy. Three impulses, of variable intensity spanning a temporal range from 40 micron sec to .3 sec will be individually studied in combination with various intensities and band widths of a background continuous noise. Chinchilla will be used as the experimental animal. Noise-induced temporary and permanent threshold shifts will be measured at regular intervals after exposure using the auditory evoked response measure. At thirty days after exposure, the chinchilla will be sacrificed, their cochleas will be infused with Araldite and flat preparations will be dissected. Analysis of the cochlear cell populations will be accomplished using phase microscopy and thin sectioning techniques. At interesting locations, the tissue will be analyzed with the electron-microscope. The final evaluation of the particular noise exposure will depend on a correlation of the noise parameters with the audiological and histological changes. These data are a necessary prerequisite for the ultimate objective of this research project, namely, the establishment of a Damage Risk Criteria for impulse-continuous noise combinations.